It's the same as, "Where are you going?"

Oh my bad. It should be to instead of too. Thank you!

Nice catch, Paul! Don't know how I missed the misuse of "too"... Whoops...

It should be spelt 'to'. You pronounce 'to' and 'too' in the same way but the meaning is different. As has been said below, 'where are you off to?' is another way of saying 'where are you going?'.